|Themes > Science > Earth Sciences > Oceanography > Ocean Sediments > Evaporites|
Evaporites-form when sea water evaporates; often at divergent margins as the process is just changing from continental rifting to seafloor spreading. In the image below, you can see a typical evaporite. It doesn't look like table salt yet, because it has red clay and black organic matter in it.
With the question of dolomite formation, we have entered the evaporite
environment. Within continental margins, the bulk of hydrogenous sediments
are evaporitic salt. Strictly speaking, calcareous shells and skeletons
are also hydrogenous, since they originate in the water. However, we have
called the minerals precipitated by organisms biogenous, and set them
3.8.1 Marine Evaporites are those sedimentswhich form on evaporation of seawater. Restriction of exchange with the open ocean, in a semi?enclosed basin, is necessary to drive the salt content high enough for precipitation to begin. Such restricted bodies of water are (1) coastal lagoons; (2) salt seas on the shelves; or (3) early rift oceans in the deep sea. A special case is the Mediterranean, which was partially isolated in the latest Miocene, 6 to 5 million years ago (see Sect. 9.5.2).
How much salt can be produced by evaporating a 1000m high column of sea water? Salt constitutes 3.5 % (or 35 0/00) of the weight of the column, its density is about 2.5 times that of water. Thus, we would obtain about 14 m salt. Most of this would be table salt (halite) (see Table 3.1). The least soluble salts precipitate first: calcium carbonate (aragonite), and calcium sulfate (gypsum). To precipitate halite, the brine needs to be concentrated about tenfold. Many evaporites only contain carbonate and gypsum (or anhydrite), others have thick deposits of halite or, rarely, of the valuable (and very soluble) potassium salts.
This sequence of mineral precipitation was experimentally established
by Usiglio in 1849.
Only very few modern examples for submarine evaporite formation are
known: coastal lagoons of Ojo de Liebre in Baja California (Mexico) or the
20km long drowned river valley Bocana de Virrila (Peru), where gypsum is
precipitated when salinities reach 160 0/00 by
evaporation and halite at more than 320 0/00 (type a
in Fig. 3.11).